journal article
Open Access
Jan 01, 2025
Axonal structure-function relationships across experimental modalities
Abstract
Abstract
The structure-function relationship of myelinated nerve fibers (Waxman & Bennett, 1972) links axon diameter and myelin thickness, commonly expressed as the g-ratio, to conduction velocity via saltatory mechanisms. Here, we investigated this relationship in the transcallosal motor pathway of the rat brain by combining functional and structural metrics in the same animals. Transcallosal conduction times (TCTs) were measured using local field potentials (LFPs) evoked by optogenetic stimulation of excitatory neurons in the motor cortex. Conduction velocity estimates were obtained by combining TCTs with transcallosal tract lengths derived from diffusion MRI (dMRI)-based tractography. Fluorescent labeling of the viral optogenetic construct verified the tractography trajectories. In parallel, axon diameter and g-ratio were quantified using both dMRI and transmission electron microscopy (TEM). To assess dehydration-induced tissue shrinkage associated with conventional TEM preparation (“Epon-TEM”), we also performed cryo-fixation followed by TEM (“Cryo-TEM”) in a separate group. This revealed diameter-dependent axonal shrinkage, yielding a correction factor of 37%. Shrinkage correction improved agreement between dMRI and Epon-TEM estimates, although dMRI remained biased toward larger axons. When translated via the structure-function relationship, TCTs corresponded to smaller axons near the mode of the TEM diameter distribution, while dMRI-based diameters predicted TCTs that were too short compared with the recorded LFP latencies. Altogether, our findings show that structural and functional metrics differ in their sensitivity profiles. Accounting for such modality-dependent sensitivities facilitates the investigation of structure-function relationships, advancing our understanding of how microstructure supports neural communication.
The structure-function relationship of myelinated nerve fibers (Waxman & Bennett, 1972) links axon diameter and myelin thickness, commonly expressed as the g-ratio, to conduction velocity via saltatory mechanisms. Here, we investigated this relationship in the transcallosal motor pathway of the rat brain by combining functional and structural metrics in the same animals. Transcallosal conduction times (TCTs) were measured using local field potentials (LFPs) evoked by optogenetic stimulation of excitatory neurons in the motor cortex. Conduction velocity estimates were obtained by combining TCTs with transcallosal tract lengths derived from diffusion MRI (dMRI)-based tractography. Fluorescent labeling of the viral optogenetic construct verified the tractography trajectories. In parallel, axon diameter and g-ratio were quantified using both dMRI and transmission electron microscopy (TEM). To assess dehydration-induced tissue shrinkage associated with conventional TEM preparation (“Epon-TEM”), we also performed cryo-fixation followed by TEM (“Cryo-TEM”) in a separate group. This revealed diameter-dependent axonal shrinkage, yielding a correction factor of 37%. Shrinkage correction improved agreement between dMRI and Epon-TEM estimates, although dMRI remained biased toward larger axons. When translated via the structure-function relationship, TCTs corresponded to smaller axons near the mode of the TEM diameter distribution, while dMRI-based diameters predicted TCTs that were too short compared with the recorded LFP latencies. Altogether, our findings show that structural and functional metrics differ in their sensitivity profiles. Accounting for such modality-dependent sensitivities facilitates the investigation of structure-function relationships, advancing our understanding of how microstructure supports neural communication.
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References
79
[1]
Aboitiz "The evolutionary origin of the mammalian isocortex: Towards an integrated developmental and functional approach" The Behavioral and Brain Sciences (2003) 10.1017/s0140525x03000128
[2]
Aboitiz "Fiber composition of the human corpus callosum" Brain Research (1992) 10.1016/0006-8993(92)90178-c
[3]
Alexander "Orientationally invariant indices of axon diameter and density from diffusion MRI" NeuroImage (2010) 10.1016/j.neuroimage.2010.05.043
[4]
Amirrashedi, M., Kjer, H. M., Barquin Torre, M. L., & Dyrby, T. B. (2025). From few labels to full insight: 3D semantic segmentation of brain white matter microstructures from a sparsely annotated X-ray nano holotomography dataset. bioRXiv. https://doi.org/10.1101/2025.06.08.658301
10.1101/2025.06.08.658301
[5]
Andersson "Axon morphology is modulated by the local environment and impacts the noninvasive investigation of its structure-function relationship" Proceedings of the National Academy of Sciences of the United States of America (2020) 10.1073/pnas.2012533117
[6]
Andersson "Does powder averaging remove dispersion bias in diffusion MRI diameter estimates within real 3D axonal architectures?" NeuroImage (2022) 10.1016/j.neuroimage.2021.118718
[7]
Ansel "PyTorch 2: Faster machine learning through dynamic Python Bytecode transformation and graph compilation" Proceedings of the 29th ACM International Conference on Architectural Support for Programming Languages and Operating Systems, Volume 2 (2024)
[8]
Arancibia-Cárcamo "Node of Ranvier length as a potential regulator of myelinated axon conduction speed" eLife (2017) 10.7554/elife.23329
[9]
Aravanis "An optical neural interface: In vivo control of rodent motor cortex with integrated fiberoptic and optogenetic technology" Journal of Neural Engineering (2007) 10.1088/1741-2560/4/3/s02
[10]
Assaf "Conservation of brain connectivity and wiring across the mammalian class" Nature Neuroscience (2020) 10.1038/s41593-020-0641-7
[11]
Bak "REPULSION, a novel approach to efficient powder averaging in solid-state NMR" Journal of Magnetic Resonance (San Diego, Calif.: 1997) (1997) 10.1006/jmre.1996.1087
[12]
Barazany "In vivo measurement of axon diameter distribution in the corpus callosum of rat brain" Brain: A Journal of Neurology (2009) 10.1093/brain/awp042
[13]
Barth "Comment on “Principles of connectivity among morphologically defined cell types in adult neocortex" Science (2016) 10.1126/science.aaf5663
[14]
Burcaw "Mesoscopic structure of neuronal tracts from time-dependent diffusion" NeuroImage (2015) 10.1016/j.neuroimage.2015.03.061
[15]
Caminiti "Diameter, length, speed, and conduction delay of callosal axons in macaque monkeys and humans: Comparing data from histology and magnetic resonance imaging diffusion tractography" The Journal of Neuroscience: The Official Journal of the Society for Neuroscience (2013) 10.1523/jneurosci.0761-13.2013
[16]
Caminiti "Evolution amplified processing with temporally dispersed slow neuronal connectivity in primates" Proceedings of the National Academy of Sciences of the United States of America (2009) 10.1073/pnas.0907655106
[17]
Cardoso, M. J., Li, W., Brown, R., Ma, N., Kerfoot, E., Wang, Y., Murrey, B., Myronenko, A., Zhao, C., Yang, D., Nath, V., He, Y., Xu, Z., Hatamizadeh, A., Myronenko, A., Zhu, W., Liu, Y., Zheng, M., Tang, Y.,… Feng, A. (2022). MONAI: An open-source framework for deep learning in healthcare (arXiv:2211.02701). arXiv. https://doi.org/10.48550/arXiv.2211.02701
[18]
Chomiak "What is the optimal value of the g-ratio for myelinated fibers in the rat CNS? A theoretical approach" PLoS One (2009) 10.1371/journal.pone.0007754
[19]
Cook, P. A., Bai, Y., Nedjati-Gilani, S., Seunarine, K. K., Hall, M. G., Parker, G. J., & Alexander, D. C. (2006). Camino: Open-source diffusion-MRI reconstruction and processing. Proceedings of the International Society for Magnetic Resonance in Medicine, 14, 2759. http://www.cs.ucl.ac.uk/research/medic/camino/files/camino_2006_abstract.pdf
10.54294/fgfrtv
[20]
Di Lazzaro "Direct demonstration of interhemispheric inhibition of the human motor cortex produced by transcranial magnetic stimulation" Experimental Brain Research (1999) 10.1007/s002210050648
[21]
Drakesmith "Estimating axon conduction velocity in vivo from microstructural MRI" NeuroImage (2019) 10.1016/j.neuroimage.2019.116186
[22]
Dyrby "An ex vivo imaging pipeline for producing high-quality and high-resolution diffusion-weighted imaging datasets" Human Brain Mapping (2011) 10.1002/hbm.21043
[23]
Dyrby "Validation strategies for the interpretation of microstructure imaging using diffusion MRI" NeuroImage (2018) 10.1016/j.neuroimage.2018.06.049
[24]
Dyrby "Interpolation of diffusion weighted imaging datasets" NeuroImage (2014) 10.1016/j.neuroimage.2014.09.005
[25]
Dyrby "Rethinking tractography and neuroanatomy: Does image resolution hold the key?" Brain Structure Function (2025) 10.1007/s00429-025-03025-0
[26]
Dyrby "Contrast and stability of the axon diameter index from microstructure imaging with diffusion MRI" Magnetic Resonance in Medicine (2013) 10.1002/mrm.24501
[27]
Fan "Axon diameter index estimation independent of fiber orientation distribution using high-gradient diffusion MRI" NeuroImage (2020) 10.1016/j.neuroimage.2020.117197
[28]
Fan "Scan-rescan repeatability of axonal imaging metrics using high-gradient diffusion MRI and statistical implications for study design" NeuroImage (2021) 10.1016/j.neuroimage.2021.118323
[29]
Ferbert "Interhemispheric inhibition of the human motor cortex" The Journal of Physiology (1992) 10.1113/jphysiol.1992.sp019243
[30]
Hoffmeyer "Nonlinear neurovascular coupling in rat sensory cortex by activation of transcallosal fibers" Journal of Cerebral Blood Flow and Metabolism: Official Journal of the International Society of Cerebral Blood Flow and Metabolism (2007) 10.1038/sj.jcbfm.9600372
[31]
Horowitz "In vivo correlation between axon diameter and conduction velocity in the human brain" Brain Structure & Function (2015) 10.1007/s00429-014-0871-0
[32]
Houzel "Morphology of callosal axons interconnecting areas 17 and 18 of the cat" The European Journal of Neuroscience (1994) 10.1111/j.1460-9568.1994.tb00585.x
[33]
Hursh "Conduction velocity and diameter of nerve fibers" American Journal of Physiology (1939) 10.1152/ajplegacy.1939.127.1.131
[34]
Innocenti "The diameter of cortical axons depends both on the area of origin and target" Cerebral Cortex (New York, N.Y.: 1991) (2014) 10.1093/cercor/bht070
[35]
Iordanova "Optogenetic investigation of the variable neurovascular coupling along the interhemispheric circuits" Journal of Cerebral Blood Flow and Metabolism: Official Journal of the International Society of Cerebral Blood Flow and Metabolism (2018) 10.1177/0271678x18755225
[36]
Jiang "Principles of connectivity among morphologically defined cell types in adult neocortex" Science (2015) 10.1126/science.aac9462
[37]
Jones "Optimal strategies for measuring diffusion in anisotropic systems by magnetic resonance imaging" Magnetic Resonance in Medicine (1999) 10.1002/(sici)1522-2594(199909)42:3<515::aid-mrm14>3.0.co;2-q
[38]
Kaden "Quantitative mapping of the per-axon diffusion coefficients in brain white matter: Quantitative mapping of the per-axon diffusion coefficients" Magnetic Resonance in Medicine (2016) 10.1002/mrm.25734
[39]
Kellner "Gibbs-ringing artifact removal based on local subvoxel-shifts" Magnetic Resonance in Medicine (2016) 10.1002/mrm.26054
[40]
Kjer "Bridging the 3D geometrical organisation of white matter pathways across anatomical length scales and species" eLife (2025) 10.7554/elife.94917
[41]
Kleinlogel "Ultra light-sensitive and fast neuronal activation with the Ca2+-permeable channelrhodopsin CatCh" Nature Neuroscience (2011) 10.1038/nn.2776
[42]
Korogod "Ultrastructural analysis of adult mouse neocortex comparing aldehyde perfusion with cryo fixation" eLife (2015) 10.7554/elife.05793.018
[43]
Kroenke "On the nature of the NAA diffusion attenuated MR signal in the central nervous system" Magnetic Resonance in Medicine (2004) 10.1002/mrm.20260
[44]
Lamantia "Cytological and quantitative characteristics of four cerebral commissures in the rhesus monkey" The Journal of Comparative Neurology (1990) 10.1002/cne.902910404
[45]
Lee "The impact of realistic axonal shape on axon diameter estimation using diffusion MRI" NeuroImage (2020) 10.1016/j.neuroimage.2020.117228
[46]
Characterization of Engineered Channelrhodopsin Variants with Improved Properties and Kinetics
John Y. Lin, Michael Z. Lin, Paul Steinbach et al.
Biophysical Journal
2009
10.1016/j.bpj.2008.11.034
[47]
Ma "Denoise magnitude diffusion magnetic resonance images via variance-stabilizing transformation and optimal singular-value manipulation" NeuroImage (2020) 10.1016/j.neuroimage.2020.116852
[48]
McDougall "Myelination of axons corresponds with faster transmission speed in the prefrontal cortex of developing male rats" eNeuro (2018) 10.1523/eneuro.0203-18.2018
[49]
Nagel "Light activation of channelrhodopsin-2 in excitable cells of Caenorhabditis elegans triggers rapid behavioral responses" Current Biology: CB (2005) 10.1016/j.cub.2005.11.032
[50]
Nilsson "Resolution limit of cylinder diameter estimation by diffusion MRI: The impact of gradient waveform and orientation dispersion" NMR in Biomedicine (2017) 10.1002/nbm.3711
Showing 50 of 79 references
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- Jan 01, 2025
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Funding
Danmarks Frie Forskningsfond
Award: 3105-00129B
Cite This Article
Christian S. Skoven, Mariam Andersson, Miren Lur Barquin Torre, et al. (2025). Axonal structure-function relationships across experimental modalities. Imaging Neuroscience, 3. https://doi.org/10.1162/imag.a.1058
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